An epitope tag is a short recognizable sequence fused to a protein so that an antibody can detect or isolate it. The FLAG tag is one example. A shared tag permits the same detection reagent to follow different engineered products. The fusion can affect folding, localization or activity, so detection of the tag does not alone establish that the tagged protein retains normal function. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 8, Antibodies as Tools for Studying Proteins
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